1. Field of the Invention
The present invention relates to an optical disc track access apparatus and method, and in particular to an optical disc track access apparatus and method which are capable of quickly accessing a destination track position on an optical disc by detecting a vibration, position variation and level variation of an objective lens and a characteristic value of an actuator when an optical pick-up moves across the tracks of an optical disc at a high speed and of controlling a tracking actuator by generating a back electromotive force corresponding to the detected values.
2. Description of the Background Art
An optical disc reproducer is an apparatus capable of reading a digital data signal recorded on an optical disc and reproducing the thusly recorded data signal. The construction of the optical disc reproducer will be explained with reference to the accompanying drawings.
FIG. 1 illustrates a conventional optical disc reproducer which includes a spindle motor 2 rotating an optical disc 1, a pick-up unit 3 reading a digital data signal recorded on the optical disc 1, a sled motor 4 moving the pick-up unit 3 in the radial direction of the optical disc 1, a motor driving unit 6 driving the motors 2 and 4, a filtering and waveform shaping unit (R/F) 5 receiving a signal detected by the pick-up unit 3 and outputting a filtered and waveform shaped signal, a traverse counter 13 receiving a signal detected by the pick-up unit 3 and counting the number of tracks of the optical disc 1 traversed by the optical pick-up unit 3, a microcontroller 11 receiving an externally inputted instruction signal IN and an output signal of the traverse counter 13, detecting the rotation speed of the sled motor 4 and the movement of the pick-up unit 3 and controlling each circuit forming the optical disc reproducer, a servo unit 7 receiving the filtered and waveform shaped signal from the filtering and waveform shaping unit(R/F) 5, a focus error signal FE and a tracking error signal TE outputted from the pick-up unit 3 in accordance with the control of the microcontroller 11 and controlling the motor driving unit 6 and a digital signal processor 8, the digital signal processor 8 recovering the filtered and waveform shaped signal into a digital signal and processing the thusly recovered signal into a compressed video signal, an MPEG unit 9 decoding the compressed video signal in accordance with the control of the microcontroller 11, and a memory 12 temporarily storing the compressed video signal.
The pick-up unit 3 includes a laser diode LD outputting a light beam having a predetermined wavelength for detecting the digital data signal recorded on the optical disc 1, an objective lens and optical devices for processing the light beam outputted from the laser diode, and a photodetector for receiving the light passed thereto through the objective lens and the optical devices and changing the thusly received light to an electrical signal.
The operation of the thusly constituted conventional optical disc reproducer will now be explained.
When the optical disc reproducer is to reproduce the signals recorded on the optical disc 1, the optical disc 1 is rotated, and the optical pick-up unit 3 is moved for thereby detecting the signals recorded on the optical disc 1 and outputting a high frequency detection signal to the filtered and waveform shaped unit (R/F) 5. The filtering and waveform shaping unit(R/F) 5 shapes the high frequency detection signal. The servo unit 7 which receives the wave-shaped signal detects a synchronous signal from the wave shaped signal, and the synchronous signal is outputted to the digital signal processor 8. The digital signal processor 8 recovers the wave shaped signal from the filtering and waveform shaping unit 5 into an “original” digital signal, and the MPEG unit 9 which receives the recovered digital signal outputs a synchronous video signal, so that the optical disc reproducer reproduces the signals recorded on the optical disc.
The operation wherein the optical disc reproducer searches the data recorded on the optical disc will next be explained.
When a user inputs an instruction signal into the microcontroller for a predetermined data to be reproduced from a track at another position based on the current track of the optical disc, the microcontroller 11 detects the position information, at which the pick-up unit 3 is currently positioned from the digital signal inputted from the digital signal processor 8 for thereby computing a track number of the optical disc corresponding to the position information detected.
At this time, the position information may be classified based on the kind of the disc. If the optical disc 1 is a compact disc(CD), the position information may be a MSB(Minute, Second, block), and if the optical disc 1 is a digital video disc(DVD), the position information may be a sector number.
After the microcontroller computes the track number of the destination track at which the information recorded on the optical disc is to be reproduced, the difference between the current track number at which the pick-up unit is currently positioned, and the computed track number is obtained, and the moving direction is determined based on a predetermined result polarity.
The microcontroller 11 which determines the number of the tracks and the moving direction multiplies the number of the movement tracks by the width of a reference track for thereby computing the destination track position to which the pick-up unit 3 is to be moved and obtaining a distance over which the pick-up unit 3 is to be moved from the position of the destination track. When the servo unit 7 controls the motor driving unit 6 so that the pick-up unit 3 is moved by a predetermined distance in accordance with the control of the microcontroller, the motor driving unit 6 outputs a driving current to the sled motor 4 for a predetermined time in accordance with a control of the servo unit 7. Thereafter, the sled motor 4 is operated, and the pick-up unit 3 is moved to the destination track position in the thusly determined moving direction.
When the pick-up unit 3 is moved to the destination track position of the optical disc and reads the track information corresponding to the moved destination track position and outputs the thusly read track information to the microcontroller 11, the microcontroller 11 judges whether the information requested by the user is the recorded destination track. As a result of the judgement, if the user's requested information is not the recorded destination track, a process whereby the track information is read from a new track position to which the pick-unit 6 is slightly moved is performed until the pick-up unit is moved to the destination track position of the optical disc.
Actually, when the optical disc reproducer searches the information recorded on the optical disc, the sled motor is driven so that the pick-up unit is positioned over the destination track position for a short time, and then the pick-up unit is moved across the tracks of the optical disc. When the pick-up unit is moved across the tracks of the optical disc, a vibration occurs in the optical devices of the pick-up unit, in particular, in the objective lens due to the inertia of the pick-up unit.
At this time, the vibration of the objective lens occurs because the physical characteristic of the pick-up unit affects the actuator which supports the objective lens. In particular, the vibration is increased when the pick-up is accelerated or decelerated. Since the vibration which affects the objective lens directly affects the actuator which controls the focus of the pick-up unit and the tracking operation, it is difficult for the actuator to focus-control the pick-up unit. In addition, since the vibration of the objective lens affects the tracking control after the pick-up unit is moved, much time is required the optical disc reproducer to search the information recorded on the optical disc.
In order to overcome the above-described problems, a conventional tracking control circuit is employed. The construction and operation of the conventional tracking control circuit will next be explained.
FIG. 2 illustrates a first example of a conventional tracking control circuit which includes a photodiode 21 receiving two sub-beams from an optical disc and converting a side spot light amount from the optical disc into electrical signals E and F, a differential amplifier 22 obtaining the difference between the electrical signals E and F and generating a tracking error signal TE(=E−F), a phase/gain compensation unit 23 compensating the phase and gain of the tracking error signal TE and outputting a driving signal, a switch 24 switching the driving signal in accordance with a switching control signal SCS from the microcontroller, a power amplifier 25 amplifying the driving signal from the switch 24, and a tracking actuator 26 receiving the amplified driving signal and adjusting the objective lens in the tracking direction and radial tracking direction of the optical disc. Here, in the tracking actuator 26, a certain distortion may occur due to the vibration of the objective lens.
In the above-described tracking control circuit, when the pick-up unit 3 is moved across the tracks at a high speed, the switching operation of the switch 25 is controlled using the switching control signal SCS outputted from the microcontroller, so that the tracking actuator 27 may not be properly operated due to a noise component generated when the optical pick-up unit moves across the tracks, thereby increasing the vibrations of the objective lens.
In order to overcome the above-described problems, as a second example of a conventional tracking control circuit disclosed in Japanese Patent Publication 7-311956(of Matsushita Electric Ind. Co. Ltd), a back electromotive force corresponding to the destination track is applied to the actuator in order to position the pick-up unit over the destination track for thereby controlling the vibration of the objective lens.
As a third example of a tracking control circuit for overcoming the above-described problems, Japanese Patent Publication 8-77572(of Matsushita Electric Ind. Co. Ltd.) discloses a technique whereby the number of tracks and the moving direction are detected, and a back electromotive force which is a predetermined off-set value corresponding to the detected moving direction and track number is applied to the actuator, so that the vibration of the objective lens of the pick-up unit is controlled.
However, in the above-described conventional tracking control circuits of the optical disc reproducer, mechanical vibrations are generated due to the inertial force generated by the wiring or actuator, and the thusly generated vibrations are increased when the optical pick-up unit is quickly accelerated or decelerated. Therefore, if the level of the vibration of the objective lens exceeds the capacity of the focus control, the focus may become blurred. Even when the focus does not become blurred, it is impossible to fully control the tracking operation, so that it takes a long time to search the information recorded on the optical disc.
In the conventional tracking control circuit of the optical disc reproducer, when the optical pick-up unit is moved over the tracks at a high speed, and when controlling the vibrations of the objective lens due to the inertia force which occurs when the optical pick-up unit is quickly accelerated or decelerated, the tracking control may exceed a predetermined limit, so that it is impossible to accurately control the focus.
Therefore, in the conventional tracking control circuit, when the objective lens is vibrated due to the inertial force, a predetermined time delay occurs from the time after the optical pick-up unit is moved until the time before the tracking servo is again under full control, so that the time required for accessing the destination track of the optical disc is increased.
In the optical disc reproducer disclosed in Japanese Patent Publications 7-311956 and 8-77572, when the optical pick-up unit is moved over the tracks of the optical disc in order to access a destination track of the optical disc at high speed, a maximum kick pulse is applied irrespective of the distance over which the optical pick-up unit is moved, and a back electromotive force which varies the kick pulses in accordance with the destination track delays the time for accessing the destination track.
In addition, in the conventional optical disc reproducer, a servo loop is needed for implementing a servo control by detecting the level of the vibration, thereby increasing the fabrication cost of the system.